Hormone

Hormones are signalling molecules secreted by endocrine glands into the bloodstream, travelling to tissues that carry the matching receptor. They come in three broad chemical families: steroid hormones such as testosterone and cortisol, peptide hormones such as insulin and growth hormone, and amino-acid-derived hormones such as thyroid hormone and adrenaline.

It matters because hormones set the background conditions for everything training and diet do — how fuel is partitioned, whether tissue is built or broken down, how hungry you feel, how well you sleep. They operate through feedback loops, which is why interfering with one rarely stays contained to that one.

In practice, most acute hormonal responses to a workout are transient and have been shown to correlate poorly with actual muscle growth. The hormonal picture that matters for results is the chronic one — sleep, energy availability, stress and thyroid function — not the spike after a set of squats.